1. Field of the Invention
This invention relates to a method for producing acrylic acid in a high yield by subjecting an acrylic acid-containing solution obtained by absorption with water to a crystallizing step thereby obtaining acrylic acid crystals and a crystal mother liquid, and circulating the crystal mother liquid to an absorption column thereby enhancing the absorption ratio of acrylic acid.
2. Description of the Related Art
Commercial production of acrylic acid generally resorts to the method of propylene oxidation which consists in subjecting propylene and/or acrolein to catalytic gas phase oxidation. When acrylic acid is produced by this method of propylene oxidation, the step of propylene oxidation gives rise to such impurities as water, acids like propionic acid, acetic acid, and maleic acid, and aldehydes like acetone, acrolein, furfural, and formaldehyde in the form of by-products. The gas containing these by-products is absorbed as an acrylic acid-containing solution generally via contact with an absorbent and obtained the solution is subsequently separated by distillation etc.
One known method, for example, produces acrylic acid of high purity by absorbing an acrylic acid-containing gas resulting from catalytic gas phase oxidation with a high boiling solvent, distilling the acrylic acid-containing solution thereby separating it into the solvent and crude acrylic acid, and subsequently subjecting the crude acrylic acid to a process of crystallization (JP-A-1997-227445). By this method, the residual acrylic acid-containing gas which has escaped absorption by a high boiling solvent is cooled and transformed into a condensate containing water, formaldehyde, and acetic acid and the condensate is discarded to the out side of system. Further, by circulating the crystal mother liquid to the absorption column, the separation of covariant components by the scrapping of a low boiling substance and the stripping of a low boiling substance is enhanced.
Certain known methods produce acrylic acid by directly crystallizing the condensate obtained by condensing an acrylic acid-containing gas (the official gazettes of International Unexamined Patent Publication JP-2000-514077, JP-2001-516736, and JP-2002-521353). When the solution is cooled at the crystallizing step in the absence of an organic solvent or an extracting agent, the crystals are solidified instead of being precipitated. Thus, an organic solvent has been hitherto used to avoid this solidification. It has been found that acrylic acid can be directly crystallized from the condensate of an acrylic acid-containing gas. These methods have been perfected based on this knowledge. They are characterized by obviating the necessity of adding a supplementary substance. The method of the JP-2000-514077 circulates part of the crystal mother liquid to the crystallizing step and the method of the JP-2001-516736 and the JP-2002-521353 resort to fractional condensation of the pertinent components in contrast to the ordinary condensation. Specifically, they separate acrylic acid as an intermediate boiling fraction, discard an acrylic acid-containing low boiling fraction as waste water, and recirculate part of an acrylic acid-containing high boiling fraction and discard the remainder thereof.
A known method produces acrylic acid by suddenly cooling an acrylic acid-containing gas as with a spray cooling device, further condensing the quenched gas thereby forming crude acrylic acid, and crystallizing the crude acrylic acid (the official gazette of International Unexamined Patent Publication 2002-539104). This method has been perfected with a view to effectively utilizing an acrylic acid oligomer which occurs during the step of acrylic acid production. It uses acrylic acid oligomer-containing bottom liquid of a separation column as a quenching liquid for an acrylic acid-containing gas, withdraws part of the quenching liquid as an effusion and decomposes the acrylic acid oligomer into acrylic acid, condenses the resultant acrylic acid, and use the resultant condensate as a quenching liquid.
A method for producing acrylic acid while embracing a process of distilling the mother liquid obtained during crystallization of an acrylic acid-containing mixture thereby obtaining a residue of distillation and a column top product and recrystallizing part of the column top product has been disclosed (the official gazette of International Unexamined Patent Publication JP-2002-519402). This method is aimed at exalting the purity of acrylic acid by distilling at least part of the mother liquid occurring during first crystallization, crystallizing the column top product formed during the distillation, returning the resultant crystals to the first crystallization, and discarding the residue of crystallization to the out side of system.
When an aqueous solution can be used as an absorbent for acrylic acid in the place of an expensive high boiling solvent, the use of this aqueous solution proves economical. Particularly, when this aqueous solution is capable of absorbing the acrylic acid-containing solution in high concentration, the use of the aqueous solution is efficient in respect that it results in decreasing the amount of treatment at the subsequent steps of purification. Thus, a method which, in absorbing in an aqueous solution an acrylic acid-containing gas formed by the reaction of catalytic gas phase oxidation, supplies the recovered water emanating from a step of azeotropic dehydration to an absorption column, supplies the acrylic acid-containing solution consequently formed to a stripping column, and obtains an acrylic acid-containing solution containing 70.9 wt. % of acrylic acid, 25.6 wt. % of water, and 2.0 wt. % of acetic acid from the bottom of the stripping column has been proposed (JP-A-2001-199931). This method obtains purified acrylic acid by subjecting the acrylic acid-containing solution to azeotropic dehydration and subsequently to a step of crystallization.
The methods disclosed in the patent documents mentioned above, however, require an expensive organic solvent and consequently entail the necessity of performing such an extra operation as re-purification of the used organic solvent (JP-A-1997-227446 and JP-A-2001-199931) or carrying out complicated fractional condensation different from ordinary condensation and liable to add to the cost of equipment. Further, since the separated fractions different from the target components are discharged from the system notwithstanding they contain acrylic acid in considerable amounts, the yield of purification incurs a decline and the fractions mentioned above demand a treatment of waste disposal (the official gazettes of International Unexamined Patent Publication JP-2001-516736, JP-2002-521353, and JP-2002-539104).
Under these existing circumstances, the development of a method for producing acrylic acid of high purity in a high yield by the use of a simple apparatus has been yearned for.